Higher operating temperatures of gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, and through the single-crystal (SX) and directional solidification (DS) methods that have been developed for these alloys. However, thermal and environmental protection is required for superalloy components if they are to operate in the hot sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBCs) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, TBCs must have low thermal conductivity, be capable of strongly adhering to the article, and remain adherent through many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between low thermal conductivity materials used to form TBCs, typically ceramic, and the superalloy materials used to form turbine engine components. For this reason, ceramic TBCs are typically deposited on a metallic bond coat that is formulated to promote the adhesion of the ceramic layer to the component while also inhibiting oxidation of the underlying superalloy. Together, the ceramic layer and metallic bond coat form what is termed a thermal barrier coating system. Typical bond coat materials are diffusion aluminides and oxidation-resistant alloys such as MCrAlY, where M is iron, cobalt and/or nickel. The aluminum content of these bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the ceramic layer to the bond coat.
Various ceramic materials have been employed as the TBC, particularly zirconia (ZrO.sub.2) stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides. These particular materials are widely employed in the art because they can be readily deposited by plasma spraying and vapor deposition techniques. A continuing challenge of thermal barrier coating systems has been the formation of a more adherent ceramic layer that is less susceptible to spalling when subjected to thermal cycling. In one form, improved spallation resistance is achieved with ceramic coatings deposited by physical vapor deposition (PVD), particularly electron beam physical vapor deposition (EBPVD), to yield a columnar grain structure characterized by gaps between grains that are oriented perpendicular to the substrate surface. A columnar grain structure promotes strain tolerance by enabling the ceramic layer to expand with its underlying substrate without causing damaging stresses that lead to spallation.
Zirconia-based thermal barrier coatings, and particularly yttria-stabilized zirconia (YSZ) coatings, produced by EBPVD to have columnar grain structures are widely employed in the art for their desirable thermal and adhesion characteristics. Nonetheless, there is an ongoing effort to improve thermal barrier coatings, particularly in terms of improved spallation resistance. One approach is to produce bond coats with relatively rough surfaces that promote adhesion of ceramic TBCs by delaying the initiation of TBC cracking caused by thermally-induced stresses. For example, bond coats deposited by air plasma spraying (APS) typically have a surface roughness of about 200 microinches (5 .mu.m) to about 500 microinches (13 .mu.m) Ra, which has been shown to significantly promote adhesion of a ceramic TBC, particularly plasma sprayed TBCs that rely on mechanical interlocking for adhesion. However, APS bond coats generally have an excessively rough surface to be compatible with EBPVD ceramic layers. On the other hand, bond coats suitable for EBPVD TBCs, such as diffusion aluminide bond coats and PVD MCrAlY overlay bond coats, do not provide adequate surface roughness for plasma sprayed TBCs.
As taught in U.S. Pat. No. 5,419,971 to Skelly et al., an alternative approach for promoting spallation resistance is to arrest the propagation of cracks along the TBC/bond coat interface by forming grooves in the surface of the bond coat or substrate. According to Skelly et al., grooves and other surface features are able to deflect the crack tip, causing it to pass through phase boundaries that impede the progress of the crack along the interface. Skelly et al. disclose various methods for forming the grooves, including the use of laser and electron beams, micromachining, abrasives, engraving and photoengraving, each of which removes material from the bond coat or substrate to form the grooves. While notable improvements in spallation resistance have been achieved with the teachings of Skelly et al., shortcomings exist, including the processing and equipment costs required for the additional step of selectively removing material to form the grooves, and limitations as to which surfaces of a component can be treated to create the grooves. In addition, this process is not performed until the part being treated is near completion, resulting in a considerable investment in the part that can be lost if a mistake occurs during the process. Accordingly, there remains a need for improved methods for producing more spall-resistant thermal barrier coatings.